Vacuum arc devices with improved arcing shields

ABSTRACT

Vacuum arc devices utilized in protecting electrical apparatus and circuits against the damaging effects of high electrical transients by circuit interruption with a high current electric arc include as an essential element thereof an arcing shield to protect insulation between oppositely poled arc electrodes. In accord with the present invention, such arcing shields are fabricated of a hard ductile gas-free carbon alloy steel containing only small amounts of additives.

Q United States Patent 11 1 1111 3,851,203

Harris 1*N0v'. 26, 1974 [54] VACUUM ARC DEVICES WITH IMPROVED 3,509,406 4/1970 Rich 313/233 ARCING SHIELDS 3,746,811 7 1973 Saito 200 144 B 3,769,538 10/1973 Harris 313/233 [75] Inventor: Lawson P. Harris, Scotia, NY.

[ Assigneer General Electric p y, Primary Examiner-James w. Lawrence h n y Assistant ExaminerWm. H. Punter 1 Notice: The portion of the term of this Attorney, Agent, or Firm-:Jerome C. Squillaro; Joseph patent subsequent to Oct. 30, 1990, Cohen; Juhus Zaskahcky has been disclaimed.

[22] Filed: July 30, 1973 [57] ABSTRACT [21] Appl. No.: 383,684 Vacuum are devices utilized in protecting electrical apparatus and circuits against the damaging effects of s CL B 3 electrical transients circuit interruption a [51] Int Cl i 19,30 d 33/66 high current electric arc include as an essential ele- [58] 313/233 146 ment thereof an arcing shield to protect insulation bei 44 tween oppositely poled arc electrodes. In accord with the present invention, such arcing shields are fabri- 56] References Cited cated of a hard ductile gas-free carbon alloy steel con- UNITED STATES PATENTS taining only small amounts of additives.

3,163,734 1 2/1964 Lee 200/144 B 8 Claims, 3 Drawing Figures 60 k 304 JT/i/NZfSS $75151 70 'Z B 60 Q a? 50 e 40 K) l- 2 0 A 4 6 8 /0 I? /4 /6 /5 20 A"? m4: Arrm cmmvr 2H0, usec PATENTEL NUV 2 6 I974 SHEET 1 BF 3 VACUUM ARC DEVICES WITH IMPROVED ARCING SHIELDS FIELD OF THE INVENTION RELATED INVENTIONS This invention is related to the inventions described and claimed in my copending applications Ser. No. 236,278 filed Mar. 20, 1972, now US. Pat. No. 3,769,538 and Ser. No. 375,133 filed June 29, 1973,

the disclosures of which are incorporated herein by reference thereto. Y

BACKGROUND OF THE INVENTION In the field of vacuum are devices, which may be of the types of vacuum gap devices, per se, triggerable vacuum gap devices, or vacuum switches, for example,

. a fixed gap device or a reclosable arcing device is utilized to protectelectrical equipment and is operative to 1 short circuit or interrupt, in-line, transient currents caused by over-voltages or line faults across a gap in vacuo. In so doing, the line current is transferred to an arc in vacuum which is extinguished and current therein is interrupted when the value of arc current falls through zero, as for example, the first occurring current zero of an alternating current cycle. Due to the high dielectric strength of vacuum, when the voltage across the separated arc-electrodes is reapplied on the next half cycle and because the arcing specie comprises metallic ions from the arc-electrode which readily diffuse and condense upon extinction of the are, the arc is not reestablished with the reestablishing of high voltage across. the primary arc-electrodes. I

In such devices, the primary arc-electrodes are connected across a high voltage and are incorporated within a unitary structure which requires an excellent high voltage electrical insulator therebetween. The insulating integrity of such high voltage insulator maintains the integrity of the device to prevent leakage currents between the opposite poles of the impressed electric voltage which is imposed across the primary arcelectrodes.

In my aforementioned applications, and in one main approach to the construction of such devices, such insulation is provided by including within the device a cylindrical sidewall member which is fabricated of a suitable high voltage insulating material many of which are well known to those skilled in the art, as for example Pyrex, Vycor, and various ceramics. In another approach to such insulation requirement, the sidewall member is metallic and all or a portion of the endwall members is constructed of such high voltage insulating material. One such structure in which all of the endwall member is constructed of an insulator is illustrated in US. Pat. No. 33,727,018 to Wesoloski et al.

Irrespective of the location of the primary high voltage insulator utilized to protect the integrity of and separate the high voltage poles of a vacuum arc device, a large shield or a plurality of shields is provided and is interposed between the gap, either permanent or reclosable, which separates the primary arc-electrodes in the open circuit position. Thus, when an electric arc is burning between the primary arc electrodes, conducting specie generally having their source in the primary arc-electrodes and therefore constituting ions of conducting metals, are spewed radially out from the arcing gap and are intercepted by the arcing shield and do not have an opportunity to deposit upon the high voltage insulator between the arc-electrodes to degrade the in tegrity thereof.

As is well known in the art, arcing shields must satisfy many of the critical criteria which must be satisfied by arc-electrodes. Thus, for example, since they are exposed'to high temperatures caused by the burning of an arc which may be conducting between 10,000 and 100,000 amperes of current for a period of up to 9% cycle or more of 60 Hz frequency power, such shields must be substantially free of gas and gas-forming materials so as not to emit gases due to the heat to which they are subjected and thus degrade the vacuum of the vacuum arc device. Additionally, such shields must be sufficiently refractory as not to deform or fracture under the intense heat and electromagnetic forces which exist in vacuum are devices. Additionally, such materials must have a relatively high voltage breakdown strength since they are often connected to one or more primary arc electrode and juxtaposed not too far from a portion of the device which is at the same potential as the other primary arc electrode or, in another type device, may be at a neutral potential but disposed relatively closed to respective members which are at the potentials of each primary arc-electrode simultaneously. In this respect, should the shield material not exhibit a high voltage breakdown strength as well as a high rate of recovery of voltage strength, the shield member can be the source of spurious arcing between the shield and one or more of the arc electrode members or other members of the device at the same potential thereof.

Finally, since shields in vacuum are devices are rather extensive and constitute a substantial part of the critical material contained within the vacuum are device, for economic considerations, in order to produce competitively priced devices, it is essential that the arcing shields be composed of materials which are as inexpensive as is possible, commensurate with the foregoing technical criteria which such shields must meet.

Accordingly, it is an object of the present invention to provide vacuum are devices suitable for protection of high voltage apparatus which contain improved vacuum arc shields.

Still another object of the invention is to provide vacuum are devices having improved vacuum arc shields which do not evolve any gas within the vacuum are device during operation and which do not create any problem of spurious arcing.

Still another object of the invention is to provide vacuum arc devices including such shields which are con- BRIEF DESCRIPTION OF THE INVENTION Briefly stated, in accord with one embodiment of the present invention, vacuum arc devices in accord therewith include an hermetically sealed envelope evacuated to a pressure of torr or less and including a pair of primary arc electrodes adapted to define an arcing gap, a primary insulator comprising a portion of said envelope and insulatingly separating the primary arc-electrodes and other members at the same potential as said electrodes from one another, and an arcing shield disposed between the arcing gap and the primary insulator of the device, which arcing shield is fabricated from a carbon alloy steel free from essentially all gases and gas-forming impurities and exhibiting high strength and ductility and high voltage breakdown strength.

The novel features characteristic of the invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood by reference to the following detailed description taken in connection with the appended drawings in which:

FIG. 1 is a schematic vertical cross-sectional view of a vacuum switch constructed in accord with the present invention;

FIG. 2 is a graph illustrating the recovery voltage and rate of recovery voltage of a typical material utilized as vacuum arc shields in accord with the present invention as compared with one material commonly used as a vacuum shield member in accord with vacuum arc devices of the prior art; and

FIG. 3 is a schematic vertical cross-sectional view of a vacuum gap device constructed in accord with the present invention and constituting an alternative embodiment to the device illustrated in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION In FIG. 1, a vacuum switch device represented generally as 10, comprises an evacuable envelope ll including an insulating sidewall member 12 which is hermetically sealed to respective upper and lower endwall members 13 and by means of metallic flanges l5 and 16, respectively, which flanges are suitably welded, brazed, or otherwise affixed to the inboard sides of endwall members 13 and 14 and embedded in suitable matching thermal coefficient of expansion seal within the respective ends of cylindrical insulating sidewall member 12. A pair of primary arc-electrodes l7 and 18 which may define an arcing gap 19 therebetween when in open circuit position are supported upon respective arc-electrode support members 20 and 21. Electrode support member 20 is fixed and is electrically and mechanically afiixed affixed to metallic endwall member 13. Electrode support member 21 is reciprocably movable through an aperture 22 in endwall member 14. Vacuum integrity within envelope 11 is maintained while permitting reciprocal mobility to support member 21 by means of bellows assembly 23 afiixed at flange 24 to endwall member 14 and at flange 25 to electrode support member 21. To insure that the main conduction current during arcing and steady-state operation is not carried through bellows assembly 23, a massive flexible bus strap 27 is affixed to reciprocally movable electrode support member 21 below bellows assembly 23 and to endwall member 14.

Arcing shield means, to protect insulating cylindrical sidewall member 12 from the deposition thereon of conducting species emanating from arcing gap 19 when arc-electrodes l7 and 18 are in open circuit position and an electric arc is established therebetween, comprises an arcing shield assembly including a main arcing shield 28 and a pair of secondary arc shield members 29 and 30. Main arcing shield member 28 constitutes essentially a hollow cylindrical member with antiarcing ferrules at the longitudinal ends thereof supported from insulating sidewall member 12 by means of a flange 31 embedded therein. Auxiliary arcing shield members 29 and 30 constitute cylindrical stub members electrically and mechanically depending inwardly from endwall members 13 and 14 respectively, to a distance so as to extend auxiliary shield members 29 and 30 within the volume encompassed by primary shield member 31. The distances between the ferruled ends of primary shield member 28 and auxiliary shield members 29 and 30 is made large as compared with the maximum spacing of arching gap 19 between arcelectrodes 17 and 18 at the extreme end of the reciprocal motion of movable arc-electrode 18. This is to prevent primary or spurious arcs being struck. between the shields. Notwithstanding this spacing, it is essential that the shield members be such as regards their gas freedom, their voltage withholding strength and other electrical characteristics so as to preclude the establishment of spurious arcs therebetween, particularly after the extinction of an arc and upon the reestablishment of the next occurring half cycle of an alternating voltage between opposite primary arc-electrode members.

In vacuum are devices of the prior art, arcing shields are generally fabricated from pure nickel or an alloy that is high in nickel content and which may contain other similar materials as, for example, chromium or cobalt. Thus, for example, the aforementioned US. Pat. No. 3,727,018 specifies that the shields utilized therein are preferably of an alloy of iron, nickel and cobalt, otherwise known as Kovar. Kovar, it is known, contains approximately 29 percent nickel, 18 percent cobalt, balance iron, and tends to be very gassy when arced. Other alloy steels utilized often contain chromium, as do stainless steels which contain primarily iron, nickel and chromium. In general, nickel is used as a shield in high quality developmental laboratory type vacuum arc devices because of its ready machinability, high ductility and its easy out-gassing characteristics so as to insure the absence of gas or gas-forming impurities inconsistent with the required 10" torr, or better, for most vacuum are devices. The cost of nickel, however, is prohibitively high for mass production of vacuum arc devices and pure nickel is not widely used in commercial devices.

The above-identified types of nickel alloys which may contain up to 20-25 percent nickel are a compromise between the easy-workable characteristic of nickel and the economics of the production of economically competitive commercial devices. Nickel, cobalt and chromium, when mixed with iron in quantities aggregating approximately 25 weight percent, form alloys which are less expensive but are satisfactorily machinable and formable for vacuum arc shield members. Cetain of these alloy steels, particularly those specifically identified as stainless steels, which generally contain approximately 25 percent of the combination of chromium and nickel, may be purchased in batches as staple items of commerce which are substantially corrosion-free and may be handled easily in factory operations without rusting or the formation of other corrosion thereupon. This is not a substantial advantage, for

the handling of material for use in vacuum arc devices should be careful and not such as would permit rusting or other corrosion in any circumstances.

Unfortunately, however, when vacuum arc shield members are fabricated from such commercial type readily available steels, such as 304 stainless steel, for

example, which contains 18 percent chromium, 8 per-.

cent nickel, minor additions of titanium, chromium and vanadium, balance iron, such materials are only marginally acceptable insofar as gas and gas-forming impurity freedom is concerned. Additionally, due to the presence of high proportions of the chromium, nickel and other additives, which are relatively expensive, such steels increase the cost of vacuum arc devices when included therein as shield members in commercial production.

In my above-identified co-pending applications, Ser. No. 236,278 and Ser. No. 375,133, there are disclosed and claimed unique, novel, and improved arc-electrode structures fabricated from ferrous materials which are hard, ductile, and uniquely adapted for fabrication in vacuum are devices to provide devices having higher recovery voltage and higher rate of recovery voltage than devices of the prior art. Although certain of these materials contain nickel, chromium, cobalt or other such transition metals and are, therefore, subject to the same economic disadvantages as conventional vacuum arc shields of the prior art, I find that certain of these materials are uniquely adapted and suitable to provide improved vacuum arc shields which may be fabricated with ease and which are substantially less expensive so as to provide a competitive advantage in commercial vacuum arc devices.

Thus, in accord with the present invention, ferrous alloy materials comprising certain carbon alloy steels have great advantage as vacuum arc shields. Carbon alloy steelsas, for example, VASCOJET 1000 CVM available from Vanadium Alloy Steel Company, Latrobe, Pa., and composed of approximately 0.40 percent. carbon, 5.0 percent chromium, 1.3 percent molybdenum, and 0.5 percent vanadium, remainder iron, typically are hardened by a quench from a high temperature of the order of l,800-1900F to form martensitic steel and later tempered at a temperature of approximately 1,000F which is of the order to magnitude at which vacuum are devices are sealed during processing and baked out to remove absorbed gases from the constituents thereof. This tempering causes the conversion of a portion of the martensite to pearlite, slightly decreasing the hardness of the carbon alloy steels and increasing the ductility in order to render the shield members more readily able to withstand the destructive heat and mechanical shock developed during operation of devices in accord with the invention. Such suitable carbon alloy steels contain approximately 0.2 to 0.5 weight percent carbon. Less than 0.2 percent carbon precludes suitable hardening, and more than 0.5 percent carbon precludes the achievement of necessary workability. Steels used in accord with the invention also contain minor additives of one or more of chromium, molybdenum, vanadium, manganese, silicon and nickel. These materials are present in an aggregate quantity not exceeding 10 weight percent of all, and not exceeding 5 percent of any single additive, are used in order that the crystalline structure of the steels used to form shields in devices of the invention be such as to permit the necessary combination of hardness and ductility required.

In addition to the aforementioned VASCOJET 1000 CVM carbon alloy steel, other vacuum melted carbon alloy steels suitable for shield members in devices in accord with the invention include AlSl-1722A containing 0.45 percent carbon, 0.95 percent chromium, 0.55 percent molybdenum, 0.30 percent vanadium, and 0.55 percent manganese, balance iron. Another suitable vacuum melted steel is AlSl 4340 containing 0.4 percent carbon, 0.8 percent chromium, 0.25 percent molybdenum, 1.8 percent nickel, 0.75 percent manganese, and 0.28 percent silicon, balance iron. Other vacuum melted carbon alloy steels satisfying the criteria set forth herein, are readily available to those skilled in the art.

As used herein, and in the appended claims, the terms hardness", hard, and the like are intended to connote a tensile strength in excess of 100,000 psi, and preferably in excess of 150,000 psi and a Rockwell C value of at least approximately 30, and preferably approximately 40. As used herein, the terms high ductility, ductile, and the like, are meant to connote a ductility as evidenced by a percentage elongation of a standard two inch long sample of standard (ASTM) ductility test cross-section of at least 5 percent, and preferably 10 percent. All percentages of compositions are expressed by weight percent.

I Carbon alloy steels utilized in accord with the present invention must necessarily be free of all gas-forming and gaseous impurities so as not to adversely affect a vacuum of approximately 10" torr, or better, required in devices in accord with the invention when, as shield members, such materials are subjected to the influence of a high current arc which may have a plasma temper ature of approximately 5000C. Such a purity is evidenced by an inclusion therein of 10 parts or less of all gas and gas-forming materials. This requisite freedom from gaseous and gas-fonning materials is achieved in the fabrication of carbon alloy steels in accord with the present invention by vacuum melting, and preferably by consumable electrode vacuum melting, wherein the steel is made an electrode of an arc struck in vacuum which progressively melts such portion of the ingot under a high vacuum so that all gas and gas-forming constituents are removed therefrom by the combination of melting and the vacuum ambient utilized therein. Such vacuum melting, particularly of a consumable electrode type vacuum melting to cause the removal of all gas and gas-forming constituents before formation of shield members therefrom, is an inexpensive method of preparation of highly gas-free metal for vacuum arc device shields. Such technique is of relatively recent commercial availability, which may account for the substantially uniform use of nickel alloy materials with high nickel and other additive content in vacuum arc devices of the prior art. Additionally, there is a general belief among those active in vacuum arc device work to the effect that carbon-bearing materials may be inappropriate for use in vacuum are devices.

To the contrary, I have found that certain carbon alloy steels, as described herein, utilized as vacuum arc electrodes inaccord with the invention of my aboveidentified applications have functioned satisfactorily to repeatedly hold off line voltages of up to 100,000 volts when devices as illustrated in FIG. 1 are in open circuit position and have routinely, in closed circuit position,

successfully carried 33 kiloamperes peak current and have interrupted the same with a high current are with an approximately 50 volts voltage drop which has routinely been extinguished at a first occurring current zero with no deleterious effects. That such materials are suitable as arc-electrodes, clearly evidences their suitability for arcing shields in vacuum arc devices.

As indicated hereinbefore, one advantage of the utilization of carbon alloy steels, as described herein for shield members in accord with the present invention, is due to the unique dielectric recovery strength and rate of recovery of such materials, believed to be a function of the hardness and gas freedom of such carbon alloy steels.

FIG. 2 of the drawing illustrates a typical plot of recovery strength in kilovolts plotted as a function of time after arcing of gas-free electrodes fabricated of VASCOJ ET 1000 CVM carbon alloy steel, as described herein as compared with identical electrodes fabricated from 304'type stainless steel which contains approximately 18 percent chromium, 8 percent nickel, minor other impurities, and the balance iron, which tests were conducted following a 250 ampere shaped current pulse utilized in vacuum are devices, as in the present invention. It naturally follows that if such materials display improved characteristics when utilized as vacuum arc-electrodes, the same characteristics will be available when the materials are utilized as shield mate rials with respect to which the requirements, other than the actual carrying of the vacuum arc and the provision of conducting specie therefor, are essentially the same.

In FIG. 2, curve A represents the recovery voltage of arc-elecrodes constructed of VASCOJET 1000 CVM carbon alloy steel after an initial interruption, as described herein. Curve B, on the other hand, represents a similar characteristic for vacuum arc-electrodes of 304 stainless steel after interruption as described herein. From Curves A and B, it is evident that the vacuum melted carbon alloy steel of Curve A exhibits a more rapid rate of voltage recovery (i.e., l5 kV/microseconds vs. 12 kVmicroseconds) and recovers to a higher value of total voltage (i.e., approximately 95 kV vs. approximately 70 kV) than the 304 stainless steel, typical of conventional commercially utilized vacuum arc shields in vacuum are devices of the prior art.

Thus, due to the vacuum-melted characteristic of the carbon alloy steels utilized and the unique combination of hardness and ductility exhibited by carbon alloy steels in accord with the present invention, a higher voltage hold-off strength may be ascribed to shield materials in accord with the present invention, in addition to which, devices constructed in accord with the present invention and utilizing shields constructed of carbon alloy steels, as set forth herein, are substantially less expensive and more readily fabricated than devices of the prior art.

Although the shield materials in accord with the present invention may be advantageously utilized in vacuum are devices containing cuprous or copper electrodes, as is conventional prior to the inventions as set forth in my above-identified co-pending applications, such shield structures are uniquely and particularly useful when utilized in devices containing primary arcelectrodes as set forth in my above-identified copending applications, particularly those fabricated from VASCOJET 1000 CVM, for example, containing 0.4 percent carbon, 5.0 percent chromium, 1.3 percent molybdenum, 0.5 percent vanadium, remainder iron. Such a vacuum arc device possesses the unique combination of high, ductile, ferrous arc electrodes and inexpensive gas-free, ductile, ferrous shield structures which are electrically superior and extremely inexpensive as compared with devices utilizing arc-electrodes of a cuprous material and arcing shields of nickel, stainless steel, or nickel iron alloys, as generally utilized in vacuum arc devices of the prior art.

FIG. 3 of the drawing illustrates an alternative em bodiment of the invention wherein a triggerable vacuum gap device is utilized in accord with the present invention. The device of FIG. 3, which is similar to the device of FIG. 1, utilizes like legends to identify like elements.

In FIG. 3, vacuum gap device 40 contains an evacuable envelope 11 including an insulating sidewall member 12, respective endwall members 13 and 14 sealed in hermetic seal to insulating member 12 with sealing flanges l5 and 16, and containing fixed, spaced arcelectrodes 17 and 18 defining a fixed interelectrode gap 19. Arc-electrodes l7 and 18 are suspended upon arc support members 20 and 21, respectively, and .a trigger electrode assembly 41 is included within the central portion of arc-electrode 18 and arc-electrode support member 21 and is adapted to provide a conducting electron-ion plasma when a voltage is applied between arc-electrode member 18 and trigger anode- 42 of trigger electrode assembly 41 as, for example, by means of trigger voltage source represented generally by battery 43 and switch 44. Shield members 28, 29, and 30 of device 40 are fabricated of carbon alloy steels essentially as the like shield members of vacuum switch 10 illustrated in FIG. 1.

The operation of triggerable vacuum gap devices is well known to those skilled in the art and need not be described herein. The advantages described with respect to the carbon alloy steel shield assemblies of the device of FIG. 1 are the same as in the device of FIG. 3.

While certain characteristics of the materials utilized in devices in accord with the invention are disclosed as vacuum arc electrodes in my aforementioned copending applications, the unique advantages thereof, particularly the economic advantage gained by using carbon alloy steels as described herein as vacuum arc shield members, are not.

While the invention has been set forth with respect to particular embodiments and specific examples thereof, many modifications and changes will readily occur to those skilled in the art. Accordingly, by the appended claims, it is intended to cover all suchmodifications and changes as fall within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent is:

l. A vacuum arc device adapted to sustain a high current are between a pair of spaced arc-electrodes during arcing and comprising:

an hermetically sealed envelope evacuated to a pressure of 10 torr or less and including, a,. a cylindrical sidewall member and a pair of oppositely disposed endwall members in vacuum-tight seal to define a reaction chamber,

a at least a portion of at least one of said members comprising a high voltage insulator capable of providing electrical insulation of high integrity between the central portions of said endwall members; b. a pair of primary arc-electrodes inwardly depending from the central portion of said endwall members and adapted to define therebetween a primary arcing gap and to sustain a high current are across said gap; andc. shield means disposed within said envelope between said arcing gap and said high voltage insulator and effective to prevent conducting material ejected from said gap during arcing from depositing upon said high voltage insulator, c said shield means being fabricated from a vacuum refined carbon alloy steel containing 0.2 to 0.5 weight percent carbon and no more than 5 weight percent of any single metallic additive and no more than weight percent of all additives thereinexhibiting a hardness as evidenced by a Rockwell C value of at least 30 and a tensile strength of at least 100,000 psi, a ductility of at least 5 percent elongation of a standard 2 inch long sample of standard ductility test cross section and having substantial freedom from sorbed gases and gas-forming impurities therein so as to permit its use in close proximity to said areelectrode while said arc-electrodes sustain arcing current densities v of approximately 200 amperes/cm for a half cycle of power alternating voltage without the emission of any substantial quantity of gaseous material inconsistent with continued maintenance of said low pressure after arcing,

0 said shield means further characterized by a voltage holdoff strength of between approximately and kilovolts and a rate of recovery after arcing of approximately 15 kilovolts per microsecond.

2.-The vacuum arc device of claim 1 wherein said shield member material contains at least one additive in addition to carbon and selected from the group consisting of chromium, molybdenum, vanadium, manganese, nickel and silicon.

' 3. The vacuum arc device of claim 2 wherein said high voltage insulator comprises an insulating sidewall member as a part of said are device and said shield member includes at least a primary hollow cylindrical member interposed between said primary arcelectrodes and a major portion of said sidewall memher.

4. The vacuum arc device of claim 3 wherein at least one auxiliary cylindrical shield member is supported from one of said endwall members and shields a portion of said high voltage insulator not shielded by said primary hollow cylindrical shield member.

5. The vacuum arc device of claim 4 in which one of said auxiliary shield members depends from each of said endwall members.

6. The device of claim 2 wherein said device is a vacuum gap device and both of said primary arc electrodes are fixed to define a fixed arcing gap.

7. The device of claim 6 wherein electrode-ion emitting trigger electrodes means are disposed within said envelope to initiate breakdown of said arcing gap.

8. The device of claim 2 wherein one of said arc electrodes is mounted upon a reciprocally movable support to move said are electrode in and out of contact with the other of said are electrodes. 

1. A vacuum arc device adapted to sustain a high current arc between a pair of spaced arc-electrodes during arcing and comprising: a. an hermetically sealed envelope evacuated to a pressure of 10 5 torr or less and including, a1. a cylindrical sidewall member and a pair of oppositely disposed endwall members in vacuum-tight seal to define a reaction chamber, a2. at least a portion of at least one of said members comprising a high voltage insulator capable of providing electrical insulation of high integrity between the central portions of said endwall members; b. a pair of primary arc-electrodes inwardly depending from the central portion of said endwall members and adapted to define therebetween a primary arcing gap and to sustain a high current arc across said gap; and c. shield means disposed within said envelope between said arcing gap and said high voltage insulator and effective to prevent conducting material ejected from said gap during arcing from depositing upon said high voltage insulator, c1. said shield means being fabricated from a vacuum refined carbon alloy steel containing 0.2 to 0.5 weight percent carbon and no more than 5 weight percent of any single metallic additive and no more than 10 weight percent of all additives therein exhibiting a hardness as evidenced by a Rockwell C value of at least 30 and a tensile strength of at least 100,000 psi, a ductility of at least 5 percent elongation of a standard 2 inch long sample of standard ductility test cross section and having substantial freedom from sorbed gases and gas-forming impurities therein so as to permit its use in close proximity to said arc-electrode while said arcelectrodes sustain arcing current densities of approximately 200 amperes/cm2 for a half cycle of power alternating voltage without the emission of any substantial quantity of gaseous material inconsistent with continued maintenance of said low pressure after arcing, c2. said shield means further characterized by a voltage holdoff strength of between approximately 90 and 100 kilovolts and a rate of recovery after arcing of approximately 15 kilovolts per microsecond.
 2. The vacuum arc device of claim 1 wherein said shield member material contains at least one additive in addition to carbon and selected from the group consisting of chromium, molybdenum, vanadium, manganese, nickel and silicon.
 3. The vacuum arc device of claim 2 wherein said high voltage insulator comprises an insulating sidewall member as a part of said arc device and said shield member includes at least a primary hollow cylindrical member interposed between said primary arc-electrodes and a major portion of said sidewall member.
 4. The vacuum arc device of claim 3 wherein at least one auxiliary cylindrical shield member is supported from one of said endwall members and shields a portion of said high voltage insulator not shielded by said primary hollow cylindrical shield member.
 5. The vacuum arc device of claim 4 in which one of said auxiliary shield members depends from each of said endwall members.
 6. The device of claim 2 wherein said device is a vacuum gap device and both of said primary arc electrodes are fixed to define a fixed arcing gap.
 7. The device of claim 6 wherein electron-ion emitting trigger electrodes means are disposed within said envelope to initiate breakdown of said arcing gap.
 8. The device of claim 2 wherein one of said arc electrodes is mounted upon a reciprocally movable support to move said arc electrode in and out of contact With the other of said arc electrodes. 